3,735 research outputs found
- and -spin relaxation time limitations of phosphorous donor electrons near crystalline silicon to silicon dioxide interface defects
A study of donor electron spins and spin--dependent electronic transitions
involving phosphorous (P) atoms in proximity of the (111) oriented
crystalline silicon (c-Si) to silicon dioxide (SiO) interface is
presented for [P] = 10 and [P] =
10 at about liquid He temperatures (
). Using pulsed electrically detected magnetic
resonance (pEDMR), spin--dependent transitions between the \Phos donor state
and two distinguishable interface states are observed, namely (i) \Pb centers
which can be identified by their characteristic anisotropy and (ii) a more
isotropic center which is attributed to E defects of the \sio bulk
close to the interface. Correlation measurements of the dynamics of
spin--dependent recombination confirm that previously proposed transitions
between \Phos and the interface defects take place. The influence of these
electronic near--interface transitions on the \Phos donor spin coherence time
as well as the donor spin--lattice relaxation time is then
investigated by comparison of spin Hahn--echo decay measurements obtained from
conventional bulk sensitive pulsed electron paramagnetic resonance and surface
sensitive pEDMR, as well as surface sensitive electrically detected inversion
recovery experiments. The measurements reveal that both and of
\Phos donor electrons spins in proximity of energetically lower interface
states at K are reduced by several orders of magnitude
Two-phase stretching of molecular chains
While stretching of most polymer chains leads to rather featureless
force-extension diagrams, some, notably DNA, exhibit non-trivial behavior with
a distinct plateau region. Here we propose a unified theory that connects
force-extension characteristics of the polymer chain with the convexity
properties of the extension energy profile of its individual monomer subunits.
Namely, if the effective monomer deformation energy as a function of its
extension has a non-convex (concave up) region, the stretched polymer chain
separates into two phases: the weakly and strongly stretched monomers.
Simplified planar and 3D polymer models are used to illustrate the basic
principles of the proposed model. Specifically, we show rigorously that when
the secondary structure of a polymer is mostly due to weak non-covalent
interactions, the stretching is two-phase, and the force-stretching diagram has
the characteristic plateau. We then use realistic coarse-grained models to
confirm the main findings and make direct connection to the microscopic
structure of the monomers. We demostrate in detail how the two-phase scenario
is realized in the \alpha-helix, and in DNA double helix. The predicted plateau
parameters are consistent with single molecules experiments. Detailed analysis
of DNA stretching demonstrates that breaking of Watson-Crick bonds is not
necessary for the existence of the plateau, although some of the bonds do break
as the double-helix extends at room temperature. The main strengths of the
proposed theory are its generality and direct microscopic connection.Comment: 16 pges, 22 figure
ISLES: Probing Extra Dimensions Using a Superconducting Accelerometer
In string theories, extra dimensions must be compactified. The possibility that gravity can have large radii of compactification leads to a violation of the inverse square law at submillimeter distances. The objective of ISLES is to perform a null test of Newton s law in space with a resolution of one part in 10(exp 5) or better at 100 microns. The experiment will be cooled to less than or equal to 2 K, which permits superconducting magnetic levitation of the test masses. To minimize Newtonian errors, ISLES employs a near null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 100 microns. The signal is detected by a superconducting differential accelerometer. A ground test apparatus is under construction
Comparison of coherence times in three dc SQUID phase qubits
We report measurements of spectroscopic linewidth and Rabi oscillations in
three thin-film dc SQUID phase qubits. One device had a single-turn Al loop,
the second had a 6-turn Nb loop, and the third was a first order gradiometer
formed from 6-turn wound and counter-wound Nb coils to provide isolation from
spatially uniform flux noise. In the 6 - 7.2 GHz range, the spectroscopic
coherence times for the gradiometer varied from 4 ns to 8 ns, about the same as
for the other devices (4 to 10 ns). The time constant for decay of Rabi
oscillations was significantly longer in the single-turn Al device (20 to 30
ns) than either of the Nb devices (10 to 15 ns). These results imply that
spatially uniform flux noise is not the main source of decoherence or
inhomogenous broadening in these devices.Comment: 4 pages, 5 figures, accepted for publication in IEEE Trans. Appl.
Supercon
Poor survival outcomes in HER2 positive breast cancer patients with low grade, node negative tumours
We present a retrospective analysis on a cohort of low-grade, node-negative patients showing that human epidermal growth factor receptor 2 (HER2) status significantly affects the survival in this otherwise very good prognostic group. Our results provide support for the use of adjuvant trastuzumab in patients who are typically classified as having very good prognosis, not routinely offered standard chemotherapy, and who as such do not fit current UK prescribing guidelines for trastuzumab
How Do the Elderly Fare in Medical Malpractice Litigation, Before and After Tort Reform? Evidence from Texas
The elderly account for a disproportionate share of medical spending, but little is known about how they are treated by the medical malpractice system, or how tort reform affects elderly claimants. We compare paid medical malpractice claims brought by elderly plaintiffs in Texas during 1988–2009 to those brought by adult non-elderly plaintiffs. Controlling for healthcare utilization (based on inpatient days), elderly paid claims rose from about 20% to about 40% of the adult non-elderly rate by the early 2000s. Mean and median payouts per claim also converged, although the elderly were far less likely to receive large payouts. Tort reform strongly affected claim rates and payouts for both groups, but disproportionately reduced payouts to elderly claimants. We thus find evidence of convergence between the elderly and the adult non-elderly in both claim rates and payouts, which is interrupted by tort reform
Strong-field effects in the Rabi oscillations of the superconducting phase qubit
Rabi oscillations have been observed in many superconducting devices, and
represent prototypical logic operations for quantum bits (qubits) in a quantum
computer. We use a three-level multiphoton analysis to understand the behavior
of the superconducting phase qubit (current-biased Josephson junction) at high
microwave drive power. Analytical and numerical results for the ac Stark shift,
single-photon Rabi frequency, and two-photon Rabi frequency are compared to
measurements made on a dc SQUID phase qubit with Nb/AlOx/Nb tunnel junctions.
Good agreement is found between theory and experiment.Comment: 4 pages, 4 figures, accepted for publication in IEEE Trans. Appl.
Supercon
Photon Shot Noise Dephasing in the Strong-Dispersive Limit of Circuit QED
We study the photon shot noise dephasing of a superconducting transmon qubit
in the strong-dispersive limit, due to the coupling of the qubit to its readout
cavity. As each random arrival or departure of a photon is expected to
completely dephase the qubit, we can control the rate at which the qubit
experiences dephasing events by varying \textit{in situ} the cavity mode
population and decay rate. This allows us to verify a pure dephasing mechanism
that matches theoretical predictions, and in fact explains the increased
dephasing seen in recent transmon experiments as a function of cryostat
temperature. We investigate photon dynamics in this limit and observe large
increases in coherence times as the cavity is decoupled from the environment.
Our experiments suggest that the intrinsic coherence of small Josephson
junctions, when corrected with a single Hahn echo, is greater than several
hundred microseconds.Comment: 5 pages, 4 figures; includes Supporting Online Material of 6 pages
with 5 figure
Multilevel effects in the Rabi oscillations of a Josephson phase qubit
We present Rabi oscillation measurements of a Nb/AlOx/Nb dc superconducting
quantum interference device (SQUID) phase qubit with a 100 um^2 area junction
acquired over a range of microwave drive power and frequency detuning. Given
the slightly anharmonic level structure of the device, several excited states
play an important role in the qubit dynamics, particularly at high power. To
investigate the effects of these levels, multiphoton Rabi oscillations were
monitored by measuring the tunneling escape rate of the device to the voltage
state, which is particularly sensitive to excited state population. We compare
the observed oscillation frequencies with a simplified model constructed from
the full phase qubit Hamiltonian and also compare time-dependent escape rate
measurements with a more complete density-matrix simulation. Good quantitative
agreement is found between the data and simulations, allowing us to identify a
shift in resonance (analogous to the ac Stark effect), a suppression of the
Rabi frequency, and leakage to the higher excited states.Comment: 14 pages, 9 figures; minor corrections, updated reference
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